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Editorial Team
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Asked: 2026-06-05T23:55:06+00:00

I have a mongo collection which has documents with two fields fieldA and fieldB,

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I have a mongo collection which has documents with two fields “fieldA” and “fieldB”, which are timestamps and I need to get all the documents which are “(fieldB-fieldA)/6000 > 2″… So I was looking for some function in order to do this…

I saw in some posts the function “$subtract” but that seems is in mongo 2.1.x and currently I’m using 2.0.x (it’s the stable), any idea how to do this with 2.0.x? or Do I need to create a new field? or Can I only do it in the application side?

btw, I’m using the ruby mongo driver…

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  1. Editorial Team

    You can use the $where operator to specify query expressions as Javascript. In the shell:

    db.myCollection.find( { $where: “(this.fieldB – this.fieldA)/6000 > 2” } );

    You should note, however, that Javascript execution can be very slow. If this is a one time query, $where may be an ok solution. Otherwise, storing the result of the equation in a separate field is the best way to ensure that queries are timely.

    Additional information on the $where operator:
    http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-JavascriptExpressionsand%7B%7B%24where%7D%7D

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